1. Field of the Invention
The present invention generally relates to a pressure sensor and, more particularly, to the pressure sensor which can be used in association with an automobile engine control or the like.
2. Description of the Prior Art
The pressure sensor has too many applications and, as far as the automobile is concerned, the pressure sensor is used in, for example, airflow measurement for a fuel injection, control of a hydraulic fluid in an automobile suspension system and/or monitoring of an internal pressure inside a gas tank. The pressure sensor is also used in, for example, a household gas leakage alarming system.
FIG. 7 illustrates a schematic side sectional representation of the prior art pressure sensor 100. This known pressure sensor 100 includes a pressure sensor element 103 of a piezoelectric-resistance type having a cavity defined inwardly from an undersurface thereof to thereby define an upper surface region serving as a pressure receiving surface. This pressure receiving surface is provided with piezoelectric resistance elements electrically connected in a Wheatstone bridge circuit. This pressure sensor element 103 has a plurality of terminals including supply terminals fed with a power source voltage, ground terminals and output terminals and is operable to convert the pressure, applied to the pressure receiving surface, into a predetermined signal which is subsequently outputted to the outside of the pressure sensor 100 through one of electrodes 104a to 104f that is wire-bonded with the output terminals.
As shown in FIG. 7, the pressure sensor element 103 is air-tightly bonded, by the use of, for example, a die-bonding technique, to a silicon pedestal 102 having a through-hole defined therein so as to open at the pressure receiving surface. The silicon pedestal 102 functions as a cushioning material for buffering stresses developed between the pressure sensor element 103 and a header 101, to thereby protect the pressure sensor element 103. This header 101 has an opening defined in alignment with the through-hole and is air-tightly bonded to the header 101 through an Au--Si junction. The header 101 is made of a 42 alloy having a coefficient of thermal expansion which is about equal to that of the silicon pedestal 102. An external pressure guiding pipe 106 is inserted in the opening in the header 101 so that a predetermined pressure medium can be guided therethrough to the pressure receiving surface of the pressure sensor element 103. A cap 105 made of SPC (cold-rolled steel plate) and having a radially outwardly protruding flange 107 is capped onto the header 101 with the flange 107 welded thereto to establish an evacuated compartment inside the resultant package. The pressure inside the evacuated compartment provides a reference pressure for the pressure sensor element 103.
As described above, the cap 105 is capped onto the header 101 during the manufacture of the pressure sensor 100. It has, however, found that during the welding of the cap 105 to the header 101, the header 101 is susceptible to deformation because of the reason which will now be discussed.
FIGS. 8A to 8C illustrates the header 101 being deformed during the course of welding of the cap 105 to the header 101, the deformation being shown exaggerated for the sake of clarity. At the outset, within a vacuum chamber, the cap 105 is capped onto the header 101 and is kept pressed against the header 101. As shown in FIG. 8A, due to the pressure applied to the header 101 through an annular projection integral with the flange 107 of the cap 105, the header 101 is deformed to assume a generally upwardly curved configuration.
Welding of the flange 107 of the cap 105 to the header 101 takes place while the header 105 is so deformed as shown in FIG. 8A. As shown in FIG. 8B, the deformation diminishes after completion of the welding.
After the welding of the cap 105 to the header 101 both of the header 101 and the cap 105 heated during the welding undergo constriction and restore to the original shape consequent upon lowering of the temperature thereof. Since the coefficient of thermal expansion of the cap 105 made of SPC is greater than that of the header 101 made of the 42 alloy, the cap 105 constricts a substantial amount greater than that exhibited by the header 101 incident to lowering of the temperature thereof. As a result of the constriction of the cap 105, the header 101 is again deformed as shown in FIG. 8C to assume a generally downwardly curved configuration.
When the deformation of the header 101 occurring during the welding is transmitted to the pressure sensor element 103 through the silicon pedestal 102, the piezoelectric resistance of the pressure sensor element 103 changes, accompanied by an undesirable change in operating characteristics of the pressure sensor element 103. Also, depending on the magnitude of deformation of the header 101, the header 101 may separate from the silicon pedestal 102 and/or cracking may occur in at least a portion of he silicon pedestal 102. In such case, the pressure inside the evacuated compartment that is used as the reference pressure may change to such an extent that an accurate pressure measurement is no longer possible.
By way of example, let it be assumed that the known pressure sensor 10 is installed in the vicinity of an automobile engine. In such case, the cap 105 will undergo a thermal expansion during the engine being operated, but undergoes a thermal shrinkage to restore to the original shape when and after the engine is halted. Not only is the header 101 deformed following the thermal expansion and shrinkage of the cap 105, but the header 101 is also deformed during the operation of the engine under the influence of engine vibrations transmitted thereto through the external pressure guiding pipe 106.
Once the header 101 is deformed in the manner described above, an undesirable change may occur in operating characteristics of the pressure sensor element 103 and/or the silicon pedestal 102 may separate from the header 101. This in turn leads to change in pressure inside the evacuated compartment that is used as the reference pressure, to such an extent that an accurate pressure measurement is no longer possible.
To lessen the adverse influence brought on the pressure sensor element by the deformation of the header, the Japanese Patent Laid-open Publication No. 57-186138 discloses the pressure sensor in which a flexible tube is disposed between the header and the silicon pedestal. According to this publication, deformation of the header is accompanied by a corresponding deformation of the flexible tube to thereby lessen the adverse influence which would be brought on the pressure sensor element. However, because of its nature, the flexible tube undergoes a deformation in correspondence with the external pressure introduced through the external pressure guiding pipe and, therefore, the pressure inside the evacuated compartment that is used as the reference pressure is apt to change, resulting in incapability of accomplishing an accurate measurement. In addition, repeated deformation of the header may eventually result in an undesirable separation of the flexible tube from the header.